The present invention relates to novel, improved coal cleaning processes of the agglomeration type for producing coal having a low pyritic sulfur content.
In some instances, the steps of my novel process, the materials used in carrying it out, and the equipment employed may be as described in pending U.S. application Ser. No. 561,168 which was filed Mar. 24, 1975, and which is assigned to the assignee of this application. U.S. application Ser. No. 561,168 (which has since matured into U.S. Pat. No. 4,173,530 dated Nov. 6, 1979) is, therefore, hereby incorporated by reference herein.
Certain terms used herein are defined as follows:
Raw coal--a composite of coal, pyritic sulfur, and "mineral matter" (the quoted term is used herein for the sake of convenience to include other inorganic material associated with coal). In general raw coal will constitute the feedstock for a process designed to remove pyritic sulfur and mineral matter therefrom. The raw coal may be as mined with or without having been subjected to preliminary preparation; or it may be the black water from a hydrobeneficiation plant or the culm from a sludge pond, etc.
Product coal--the phase generated in and recovered from a specified cleaning process and consisting of particles which are up to 99% by weight or more coal.
Copending U.S. application which has since matured into U.S. Pat. No. 4,186,886 dated Feb. 5, 1980, Ser. No. 933,845 filed Aug. 15, 1978, and assigned to the assignee of the present application, discloses a novel process for cleaning coal which involves the steps of:
(a) comminuting raw coal in aqueous slurry and in the presence of a fluorochlorocarbon agglomerant with respect to which the coal is hydrophobic to generate to generate two phases, one composed of particles of mineral matter and the other of particles of coal having freshly exposed surfaces;
(b) mechanically forcing the particles of coal together in the slurry and in the presence of the fluorochlorocarbon to agglomerate the particles of coal and to eject water and mineral matter from the agglomerates into the aqueous phase of the slurry; and
(c) kneading or working the agglomerates to expel additional mineral matter and water therefrom.
This benefication process produces a product coal phase composed of dense agglomerants and an aqueous carrier-mineral matter phase.
The agglomeration process just described is capable of reducing the mineral matter contents of coals to levels well below even those which can be attained by employing the state of the art process disclosed in copending U.S. application Ser. No. 561,168. However, the agglomeration process is not as effective as might be desired in removing pyritic sulfur from the coal being cleaned. This is advantageous in certain cases because subsequent combustion of the coal results in the conversion of the pyritic sulfur to sulfur dioxide, creating an atmospheric pollution problem.
One primary object of the present invention resides in the provision of novel, improved coal cleaning processes which employ an agglomeration technique and which also have the capability of reducing the pyritic sulfur content of the product coal to an extremely low level.
In general this and other important objects of the invention are achieved by adding calcium oxide in either anhydrous or hydrated form to the coal being cleaned. For reasons which are not fully understood, the calcium oxide is effective in the presence of the freshly exposed, unoxidized surfaces generated in comminuting the raw coal to cause pyritic materials to remain dispersed in the aqueous phase of the slurry without adversely affecting the coalescence of the product coal. That is, the calcium oxide apparently inhibits the ability of the pyritic material to agglomerate along with product coal without effecting the agglomeration of the latter.
The process of the present invention as just described is capable of reducing the pyritic sulfur content of coal to a level which has heretofore been equalled only by non-competitive, chemical processes of coal treatment such as oxidation, gasification, and liquefaction. Pyritic sulfur contents of only a few one-hundreths of one percent have consistently been obtained.
It has heretofore been proposed in U.S. Pat. No. 3,919,080 issued Nov. 11, 1975, to Stauter, for example, that sodium sulfite be used as a pyrite depressant in coal recovery processes. This approach is inferior to the novel process described herein because the Stauter depressant increases the sodium ion concentration of the coal. As a result, the fusion point of the ash formed when the coal is burned is lowered to a level where the corrosion problems the ash causes become critical.
Furthermore, the reductions in pyrite content that can be obtained by processes using sodium sulfite as a pyritic sulfur depressant are much smaller than those I am able to achieve.
A coal upgrading process which appears at first blush to resemble mine in that lime is employed as an additive is disclosed in U.S. Pat. No. 3,637,464 issued Jan. 25, 1972, to Walsh et al. Closer inspection, howver, reveals that there is actually little similarity between the two processes. In the Walsh et al process the lime is added to an aqueous dispersion of coal after the coal has been ground to reduce its particle size. As a result, the calcium oxide is exposed only to oxidized coal particle surfaces; and it consequently cannot interact with the coal in the manner I have found essential for the minimization of pyritic sulfur in the product coal (as discussed above, this goal requires that the calcium oxide interact with freshly exposed surfaces of the raw coal particles).
Furthermore, the process disclosed in the Walsh et al patent necessarily results in a film of the oil used as a bridging agent being left on the surfaces of the product coal; and high temperatures are employed to recover that oil which is not left on the coal. This can make the process uneconomical in many instances due to the loss of the unrecovered bridging agent and the energy consumed in recovering that oil which is not left on the coal.
The foregoing are deficiencies which are remedied by my process. The latter allows the use of materials which can be recovered in essentially quantitative amounts from the product coal, and only a fraction as much energy is needed to recover the process material.
Another coal beneficiation process in which limestone can be employed as an additive is described in U.S. Pat. No. 4,033,729 issued July 5, 1977, to Capes et al. In that case, however, the additive is not employed in the manner or for the purposes I have in mind. Instead, it is used to promote the coalescence of particulate inorganic minerals in a reverse agglomeration process.
U.S. Pat. No. 4,080,176 issued Mar. 21, 1978, to Verschuur discloses a coal beneficiation process which somewhat resembles mine in that calcium hydroxide can be used as an aid to desulfurization. Otherwise, the process is quite different. It is carried out at high temperatures (preferably above 250.degree. C. (482.degree. F.)) and under very high pressure (100 atmospheres is mentioned); and the calcium hydroxide is employed only as a solubilization aid for insoluble sulfur compounds in the coal being processed, not to effect a separation of pyritic sulfur particles from product coal.
Lime has also been used as a settling agent in flotation processes and to adjust the pH of aqueous slurry feedstocks as shown, respectively, by U.S. Pat. Nos. 2,784,468 issued Mar. 12, 1957, to Booth et al. and 3,394,893 issued July 30, 1968, to Moss et al. Again, however, the lime is used in a manner and for a purpose which is quite different from that I contemplate in that there is no interaction between the additive and unoxidized coal particle surfaces.
As discussed above, the presence of sodium ions in coal is undesirable because of the corrosion problems this creates. In contrast, the presence of calcium ions can be a decided benefit. When the coal is burned, the calcium ions react with sulfur remaining in coal, forming a precipitate that can be readily removed from the combustion products. Thus, the presence of calcium ions in the coal produced by my novel process actually facilitates the removal of pollutants from the combustion products.
There is also evidence that calcium increases the hydrogasification and steam gasification reactivities of coal. My novel process has the additional advantage in this respect that the calcium is present in a relation to the coal which promotes the catalytic activity of the metal in such reactions.
One primary object of my invention has been described above.
Another important and primary object of the invention is the provision of novel, improved, coal cleaning processes of the character described in the previously set forth primary object in which the reduction of pyritic sulfur can be inexpensively affected.
Yet another important and primary object of the invention resides in the provision of coal cleaning processes in accord with the preceding objects in which a calcium oxide is employed to promote the reduction of pyritic sulfur.
Still another important and related object of the invention resides in the provision of a process in accord with the preceding object in which, in the course of the process, the calcium oxide is associated with the product coal in a manner which increases the hydrogasification and steam gasification reactivities of said coal.
Other important objects and features and additional advantages of my invention will become apparent from the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing.